the download speed was 215kb I've never had that before, and if net meter is to be believed I'm regularly in the 140-170kb range again an uncommon occurrence.
Possibly due to the hotter weather, or perhaps you banged the grey box with your mop in just the right way... don't do any more mopping!!
Seriously though, for the line attenuation, that noise margin should be a lot higher on a fixed service, and that really needs sorting before trying to upgrade the line speed at all.
As it stands, you wouldn't really benefit that much from a speed upgrade, but if you do follow the advice on the
Low SNR page, you may well make a big improvement.
Normally on a line with 28dB attenuation, you'd expect to easily achieve a stable, full speed 8Mbps connection if you got upgraded, and higher still if you went for an ADSL2+ service (Be, O2, UK Online etc), BUT not as the line is currently.
As others have said already, politely decline the upgrade offer for now, but do see if you can improve the internal wiring, as there's every possibility you'll want a faster connection at some point in the future.